In time division multiplex communications, each of a plurality of channels utilizes a common communication transmission medium during a respective one of successive time intervals. The individual channels are sampled in a predetermined sequence and the samples are transmitted in sequence over the common medium. A sequence of corresponding samples from each individual channel is denoted a frame. In pulse code modulation (PCM) systems, such samples are in the form of a digital code, typically eight bits in length, derived from a non-linear digitalization of a sample of the voice signal on the channel in accordance with a predetermined convention. In North America, a so-called Tl convention has been adopted, wherein a frame consists of samples from 24 channels. In Europe, a 32 channel system has been adopted (32 byte frame). The code (or byte) representative of the sample is typically 8 bits in length, the most significant bit of the byte being a sign bit, derived in accordance with a .mu.-law (e.g., .mu.=255) convention from a 14 bit linear digital representation of the sample, the most significant bit of the 14 also being a sign bit. Bit rates of on the order of 2.048 M bits/sec are typical.
Two party conversations are affected in a TDM system through a simple exchange of encoded speech signals, that is, the encoded samples from the respective party channels are transmitted during the time slot (interval) corresponding to the other channel. A problem, however, arises in TDM PCM systems when a conference call of three or more party channels is to be effected.
Various schemes for effecting such conference calls have been utilized in the prior art. For example, some prior art systems transmit to the respective party channels only the contents of that other party channel having the largest signal content. Examples of such systems are described in U.S. Pat. Nos. 3,908,089 issued Sept. 23, 1975 to Picandet and 4,007,338 issued Feb. 8, 1977 to McLaughlin. Other prior art systems, such as those described in U.S. Pat. Nos. 3,551,600 issued Dec. 29, 1970 to Berch and 3,984,643 issued Oct. 5, 1976 to Inrig, et al, accumulate the sum of all party channel samples during the course of a frame, then transmit in appropriate time slots a signal indicative of the sum of all party channel samples minus the contribution of the party channel corresponding to the particular time slot. In such systems, the samples in alternate frames are generally applied to first and second memories respectively, the previous frame being processed concurrently with the storage of the next successive frame. It should be appreciated that the accumulator is updated only upon the completion of a frame, that is, on a frame-by-frame basis.